The polyblends of the present invention are known commercially as high impact polystyrene (HIPS). The HIPS polyblends are used in large quantities in molded articles for packaging and appliances. The reinforcing rubber phase gives improved toughness but acts as a filler reducing flow during molding.
The trend to deep cavity molds for injection molding and vacuum forming has made it necessary that the HIPS polyblends have ever increasing melt flow without decreasing toughness or tensile strength in the molded parts. Plasticizers and lubricants have been studied to increase the flow of the rigid matrix phase but loss of toughness and tensile strength have placed serious limitations on this approach to the problem.
U.S. Pat. No. 3,506,740 has disclosed the use of polymeric plasticizers such as polybutylene and polypropylene. However, their incompatibility in the polyblend destroys transparency needed for many packaging and appliance uses.
U.S. Pat. No. 3,907,930 has disclosed the use of block copolymers comprising 55 to 90 percent by weight of a monovinylidene aromatic monomer and about 1-45 percent by weight of a diene monomer. Although highly functional in increasing melt flow such copolymers are commercially expensive, hence, raise the cost of the polyblends.
It has now been discovered that low-cost polyethylene waxes formed during polyethylene polymerization are highly functional in increasing the melt flow of HIPS polyblends, yet unexpectedly do not lower the tensile strength or toughness of the formed polyblend parts.